As plant diseases, there are those caused by climate, soil, or other environmental cause, those caused by a virus, bacteria, fungus (filamentous fungus), or other infective cause, those caused by a physiological disorder, and those caused by a compounded cause of the above. Even presently, there are numerous plant diseases that are impediments to production of foods, flowers, flowering trees, timbers, etc., and many of these diseases are high in economic impact.
Among plant diseases, fungi represent one of the most important pathogenic factors. It is said that approximately 80% of plant diseases are caused by fungi.
For example, blast is one of the most important plant diseases and occurs in various parts of the world. The pathogen is rice blast fungus (scientific name: “Magnaporthe oryzae”), which is a type of mold (filamentous fungus). For rice blast fungus, a suitable temperature for growth, spore formation, and infection is around 25° C. and the fungus favors humid environments. Outbreak of this fungus thus occurs due to low summer temperatures, heavy rainfall, inadequate sunlight, and other climatic causes to bring about crop failure and quality degradation of rice and apply a serious impact on economy.
There are numerous other plant diseases that are caused by fungi and are high in economic impact, such as sheath blight, rust, powdery mildew, anthracnose, sclerotinia rot, downy mildew, gray mold, etc. Development of novel agricultural chemicals, breed improvement, etc., are thus being carried out even today (in regard to control of blast, see, for example, Patent Documents 1 and 2).
Mycoviruses, which are relevant to the present invention, shall now be described.
A virus that infects fungi is called a mycovirus. Among these, mycoviruses having double-stranded RNA as a genome have been reported. Many of these mycoviruses infect host fungi latently and have hardly any impact on characteristics of the host.
Mycoviruses having a double-stranded RNA as the genome are presently classified into five families, including Partitiviridae, Totiviridae, and Chrysoviridae. Partitiviridae viruses have two straight-chain double-stranded RNAs of substantially the same size inside a virus particle and have a total gene size of 4 to 6 kbp. Totiviridae viruses have a single straight-chain double-stranded RNA of 4 to 7 kbp inside a virus particle. Besides the above, in chestnut blight fungus (scientific name: “Cryphonectria parasitica”), a virus that is present endogenously in the fungus and having a double-stranded RNA of 9 to 13 kbp has been discovered (hypovirus, etc.).
Chrysoviridae viruses have a spherical, virus-like particle and double-stranded RNAs of four components. These viruses, like Partitiviridae and Totiviridae viruses, are known to have a region encoding RdRP (RNA-directed RNA polymerase; RNA-dependent RNA polymerase, the same applies hereinafter). As examples of viruses belonging to Chrysoviridae, Hv145SV (“Helminthosporium victoriae 145S virus,” the same applies hereinafter), PcV (“Pencillium chrysogenum virus,” the same applies hereinafter), AbV1 (“Agaricus bisporus virus 1,” the same applies hereinafter), etc., are known (see Non-Patent Document 1, etc.)
Recently, with respect to specific plant diseases, methods for attenuating the pathogen by a mycovirus, etc., and using the attenuated pathogen to control the disease are being examined and have come to be put to practical use in part. For example, a method of using a full-length cDNA of viral double-stranded RNA that suppresses toxicity of chestnut blight fungus to attenuate the fungus and apply the attenuated fungus to control of chestnut blight (see Non-Patent Document 2, etc.), a method of discovering a double-stranded RNA virus that suppresses violet root rot fungus (scientific name: “Helicobasidium mompa”) and using an attenuated violet root rot fungal strain incorporating the double-stranded RNA to control violet root rot (see Non-Patent Document 3, Patent Document 3, etc.), etc., have been disclosed.
[Patent Document 1]
    Japanese Published Unexamined Patent Application No. 2004-143045[Patent Document 2]    Japanese Published Unexamined Patent Application No. 2003-250370[Patent Document 3]    Japanese Published Unexamined Patent Application No. 2001-78752[Non-Patent Document 1]    C. M. Fauquet, Mary Ann Mayo, J. Maniloff, U. Desselberger, L. A. Ball, “Virus Taxonomy: Classification and Nomenclature of Viruses; Eighth Report Of The International Committee On Taxonomy Of Viruses,” Elsevier Academic Press: pp. 591-595.[Non-Patent Document 2]    Gil H. Choi and Donald L. Nuss, “Hypovirulence of chestnut blight fungus conferred by an infectious viral cDNA.” Science. 1992 Aug. 7; 257(5071): 800-3.[Non-Patent Document 3]    H. Osaki et al, “Detection of Double-Stranded RNA Virus from a Strain of the Violet Root Rot Fungus Helicobasidium mompa Tanaka” Virus Genes 25:2, 139-145, 2002.